Network delay identification method and apparatus

ABSTRACT

Network delay is determined in order to synchronize a clock in a mobile station with a reference clock. Tones are generated that represent a sequence of bits in a synchronization flag. The tones are sampled beginning at a selected sample start time. The sampled tones are demodulated to identify the bit values in the synchronization flag. The demodulator is synchronized with the sampled tones in the synchronization flag by shifting the sample start time until the samples generate an optimum synchronization value. A reference time is then identified according to the optimum sample start time.

RELATED APPLICATIONS

[0001] This application claims priority from U.S. patent applicationSer. No. 09/602,593, filed Jun. 22, 2000, which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

[0002] Portable devices often need to be synchronized with a referencesystem. For example, portable Global Positioning Systems (GPS), deriveposition data from signals transmitted from satellites. The portable GPSdevice uses a clock to obtain a position fix with a GPS satellite. Whenthe GPS signals are not available, for example, because the satellitesare out of view, or when the mobile device has not acquired thesatellite signal, the mobile device must be resynchronized with areference clock due to drift in the mobile unit clock over time.

[0003] Network delay is defined as the time required to sendsynchronization information over a network between a reference stationand a remote station. In order to accurately resynchronize a remoteclock with a reference clock, it is necessary to account for thisnetwork delay.

[0004] This patent application describes a system for resynchronizing aclock in a portable device by sending a pulse from a reference stationto the portable device. The portable device relays a pulse back to thereference station. The reference station measures the time differencebetween the transmitted pulse and the received pulse. This timedifference is then used to resynchronize the clock in the portabledevice.

[0005] The present invention provides another technique for identifyingnetwork delay.

SUMMARY OF THE INVENTION

[0006] Network delay is determined in order to synchronize a clock in amobile station with a reference clock. Tones are generated thatrepresent a sequence of bits in a synchronization flag. The tones aresampled beginning at a selected sample start time. The sampled tones aredemodulated to identify the bit values in the synchronization flag. Thedemodulator is synchronized with the sampled tones in thesynchronization flag by shifting the sample start time until the samplesgenerate an optimum synchronization value. A reference time is thenidentified according to the optimum sample start time.

[0007] The foregoing and other features and advantages of the inventionwill become more readily apparent from the following detaileddescription of preferred embodiments of the invention, which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a diagram showing a wireless communications network thatprovides in-band signaling (IBS) according to the invention.

[0009]FIG. 2 a detailed diagram of a cellular telephone coupled to anIBS modem according to one embodiment of the invention.

[0010]FIG. 3 is another embodiment of the IBS modem according to theinvention.

[0011]FIG. 4 is a detailed diagram of an IBS modem encoder.

[0012]FIG. 5 is a schematic diagram of a IBS packet.

[0013]FIG. 6 is a schematic diagram of digital data tones output from aIBS modulator.

[0014]FIG. 7 is a diagram showing how digital data is corrupted by anAutomatic Gain Controller.

[0015]FIG. 8 is a diagram showing how a digital wireless network canfilter out digital data tones.

[0016]FIG. 9 is a detailed diagram of receiving circuitry coupled to anIBS modem decoder.

[0017]FIG. 10 is a state diagram for the IBS decoder shown in FIG. 9.

[0018]FIG. 11 is a block diagram showing a search state in the IBSdecoder.

[0019]FIG. 12 is a block diagram showing an active state in the IBSdecoder.

[0020]FIG. 13 is a block diagram showing a clock recovery state in theIBS decoder.

[0021]FIG. 14 is a schematic diagram of a cellular phone with the IBSmodem located in a detachable battery pack.

[0022]FIG. 15 are schematic diagram showing different data sourcescoupled to a cellular telephone through a IBS modem.

[0023]FIG. 16 is a schematic diagram showing implementation of the IBSmodem using a sound card.

[0024]FIGS. 17 and 18 are block diagrams showing how the sound card inFIG. 16 operates.

[0025]FIG. 19 is a block diagram of a synchronization circuit for theIBS modem.

[0026]FIG. 20 is a detailed diagram of the synchronization circuit inFIG. 19.

[0027]FIG. 21 is a timing diagram showing how the synchronizationcircuit in FIG. 19 operates.

[0028]FIG. 22 is a graph showing how the synchronization circuitidentifies the optimum synchronization start time.

[0029]FIG. 23 is an alternative implementation of the synchronizationcircuit.

[0030]FIG. 24 is an encoder diagram for a multichannel IBS modem.

[0031]FIG. 25 is an decoder diagram for a multichannel IBS modem.

[0032]FIGS. 26 and 27 show different channel configurations for themultichannel IBS modem shown in FIGS. 24 and 25.

[0033]FIG. 28 is an encoder diagram for a multicarrier IBS modem.

[0034]FIG. 29 is an decoder diagram for a multicarrier IBS modem.

[0035]FIG. 30 is a diagram of a communication system that synchronizesclocks using a synchronization flag.

[0036]FIG. 31 shows a graph of a single timing pulse used forsynchronizing a remote clock.

[0037]FIG. 32 shows a graph of a synchronization flag used forsynchronizing a remote clock.

[0038]FIG. 33 is a flow diagram showing how network delay is determinedusing the synchronization flag.

[0039]FIG. 34 is a diagram of a synchronization packet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0040] Referring to FIG. 1, a wireless communications network 12includes a cell phone 14 that receives voice signals 22 from a user 23.A voice coder (vocoder) 18 in the cell phone 14 encodes the voicesignals 22 into encoded digital voice signals 31 that are thentransmitted over a wireless digital radio channel 34 (cell call). Thecell phone 14 transmits the encoded voice signals 31 to a cellularcommunications site (cell site) 36 that relays the cell call to aCellular Telecommunications Switching System (CTSS) 38.

[0041] The CTSS 38 either connects the cell call to another cell phoneeither in the wireless cellular network 12, to a landline phone on aPSTN network 42 as a circuit switched call or routes the cell call overa packet switched Internet Protocol (IP) network 46 as a Voice Over IP(VoIP) call. The cell call can also be routed from the PSTN network 42back to the cellular network 12 or from the PSTN network 42 to the IPnetwork 46, or visa versa. The cell call eventually reaches a telephone44 that corresponds with a destination phone number originally enteredat the cell phone 14.

[0042] Additional data could be inserted at any point in the cellularnetwork 12, such as in PSTN network 42 and IP network 46 and the signalremodulated for transmission over wireline or cellular networks. Suchdata could be system related such as routing information, toll or tariffinformation, etc.

[0043] An In-Band Signaling (IBS) modem 28 enables cell phone 14 totransmit digital data 29 from a data source 30 over the radio channel 34of the cellular network 12. The IBS modem 28 modulates the digital data29 into synthesized digital data tones 26. The digital data tones 26prevent the encoding components in the cellular network 12 and landlinenetwork 42, such as vocoder 18, from corrupting the digital data. Theencoding and modulation scheme used in the IBS modem 28 allows digitaldata 29 to be transmitted through the same voice coder 18 used in thecell phone 14 for encoding voice signals 22. Any appliance such as avending machine, etc could be enhanced by this technology.

[0044] Synthesized tones are defined as signals that represent digitaldata that also have signaling characteristics that enable the signals tobe encoded and decoded by a voice codec without losing the digital datainformation in the signal. In one example, Frequency Shift Keying (FSK)signals are used to created the synthesized tones at differentfrequencies within the audio range of human speech.

[0045] The IBS modem 28 enables voice signals 22 and digital data 29 tobe transmitted over the same digital audio channel using the same cellphone circuitry. This prevents a user from having to transmit digitaldata using a separate wireless modem and enables a cell phone user totalk and send data during the same digital wireless call.

[0046] The invention modulates the digital data 29 into synthesizedaudio tones. This prevents the cell phone vocoder 18 from filtering orcorrupting the binary values associated with the digital data 29. Thesame cell phone transceiver and encoding circuitry is used fortransmitting and receiving both voice signals and digital data. Thisenables the IBS modem 28 to be much smaller, less complex and moreenergy efficient than a standalone wireless modem. In some embodiments,the IBS modem 28 is implemented entirely in software using only theexisting hardware components in the cell phone 14.

[0047] One or more servers 40 are located at any of various locations inthe wireless network 12, PSTN network 42, or IP network 46. Each server40 includes one or more IBS modems 28 that encode, detect and decode thedigital data 29 transmitted and received over the digital radio channel34. Decoded digital data is either processed at the server 40 or routedto another computer, such as computer 50.

[0048] Referring to FIG. 2, a first transmitting portion of the IBSmodem 28 includes an IBS encoder 52 and a Digital to Analog converter(D/A) 54. The IBS encoder 52 is typically implemented using a DigitalSignal Processor (DSP). The data source 30 represents any device thatrequires wireless transmission or reception of digital data. Forexample, the data source 30 can be a laptop computer, a palm computer ora Global Positioning System (GPS) (see FIG. 15).

[0049] The data source 30 outputs a digital bit stream 29 to the IBSencoder 52. The IBS encoder 52 converts the digital data 29 into LBSpackets specially formatted for transmission over a digital wirelessvoice channel. The IBS encoder 52 then converts the bits from the IBSpackets into digital data tones that are then fed into the D/A converter54.

[0050] The IBS modem 28 outputs binary values that each represent anamplitude and phase component of an audio tone. The D/A converter 54converts these digital values into analog audio tones 26 that are thenoutput to an auxiliary audio port 15 on the cell phone 14. The analogaudio tones 26 are then processed by the cell phone 14. An Analog toDigital (A/D) converter 16 in the cell phone 14 encodes the synthesizedanalog audio tones 26 into digital values. The vocoder 18 encodes thedigital representations of the synthesized tones 26 into encoded digitaldata 32 and outputs the encoded data to a transceiver 19 that transmitsthe encoded digital data 32 over the radio channel 34.

[0051] The preferred voltage of the synthesized audio tones 26 outputfrom the D/A converter 26 is around 25 millivolts peak to peak. Thisvoltage level was discovered to prevent the audio tones 26 fromsaturating the voice channel circuitry in cell phone 14.

[0052] Because the digital data 29 is fed through the existing auxiliaryhands free audio port 15 in cell phone 14, the IBS modem 28 can beinstalled as an after market device that can connect any data source 30to the cell phone 14. The data source 30 can transmit digital data 29 inany digital format. For example, the digital data 29 can be sent over anRS-232 interface, Universal Serial Bus (USB) interface, or any otherserial or parallel interface.

[0053]FIG. 3 shows an alternative embodiment of the IBS modem 28. TheIBS modem 28 in FIG. 3 is located inside the cell phone 14 and isimplemented in software using the existing cell phone processor or usingsome combination of its own components and the existing cell phonecomponents. In this embodiment, the cell phone 14 may include a dataport 56 that receives the digital data 29 from the external data source30. In an alternative embodiment, the digital data source 30 is internalto the cell phone 14. For example, the data source 30 may be a GlobalPositioning System (GPS) chip that includes a GPS receiver (not shown)for receiving global positioning data from GPS satellites (FIG. 14).

[0054] The IBS encoder 52 in FIG. 3 as mentioned above is typicallyimplemented in software using a DSP and may use the same DSP used forimplementing the vocoder 18. The D/A converter 54 outputs thesynthesized audio tones representing digital data 29 to the internal A/Dconverter 16 of the cell phone 14. The IBS encoder 52 in an alternativeembodiment, not only synthesizes the digital data 29 into audio tonesbut also quantizes the digital frequency values. The IBS encoder 52 thenoutputs the quantized data 55 directly into the vocoder 18. In stillanother embodiment of the invention, the IBS encoder 52 is implementedentirely in software in the same DSP that implements the vocoder 18.

[0055] The vocoder 18 uses a specific encoding scheme associated withthe wireless communications network 12 (FIG. 1). For example, thevocoder 18 could be a VCELP encoder that converts voice signals intodigital CDMA signals. The A/D converter 16, D/A converter 54 andtransceiver 19 are existing cell phone components known to those skilledin the art.

[0056] It is important to note that the IBS encoder 52 enables thedigital data 29 to be transmitted using the same cell phone circuitrythat transmits voice signals. The IBS encoder 52 prevents any signalapproximation, quantization, encoding, modulation, etc. performed bythe, A/D converter 16, vocoder 18, or transceiver 19 from corrupting orfiltering any bits from the digital data 29.

[0057]FIG. 4 is a detailed diagram of the IBS encoder 52 shown in FIG. 2and FIG. 3. A data buffer 58 stores the binary bit stream 29 from thedata source 30. A packetizer 60 segments the bits in buffer 58 intobytes that comprise a IBS packet payload. A packet formatter 62 adds apacket preamble and postamble that helps prevent corruption of the IBSpacket payload. An IBS modulator 64 then modulates the bits in the IBSpacket with two or more different frequencies 66 and 68 to generatedigital data tones 69.

[0058] Preventing Corruption of Digital Data in Voice Channels

[0059] Cell phone voice coders increase bandwidth in voice channels byusing predictive coding techniques that attempt to describe voicesignals without having to send all the information associated with humanspeech. If any unnatural frequencies or tones are generated in the voicechannel (i.e., frequencies representing digital data), those frequenciesmight be thrown out by the voice coder 18 (FIG. 2). For example, if theamplitude of the digital data tones are greater than that of normalvoice signals or the same digital data tone is generated for too long atime period, the voice coder 18 might filter out that high amplitude orextended frequency signal. Depending on how the digital data tones areencoded, the digital bits represented by those unnatural audio tones maybe partially or entirely removed from the voice channel.

[0060] The IBS encoder 52 encodes the digital data 29 in a manner wherevoice coders will not filter or corrupt the tones representing digitaldata. The IBS encoder 52 does this by controlling the amplitudes, timeperiods and patterns of the synthesized audio tones used to representthe binary bit values.

[0061] Referring to FIG. 5, the packet formatter 62 (FIG. 4) adds apacket preamble 73 and a header 75 to the front of a IBS packet 70. Thepacket preamble 73 includes a preamble pattern 72 and a sync pattern 74.A checksum 78 and a packet postamble 79 are attached to the backend ofthe IBS packet 70.

[0062]FIG. 6 shows the synthesized digital data tones 69 output from theIBS modulator 64 (FIG. 4). The IBS modulator 64 (FIG. 4) converts thedigital bits in the IBS packet 70 into one of two different tones. Afirst tone is generated at an f1 frequency and represents a binary “1”value and a second tone is generated at a f2 frequency and represents abinary “0” value. In one embodiment the f1 frequency is 600 Hertz andthe f2 frequency is 500 Hertz (Hz).

[0063] It has been determined that the most effective frequency rangefor generating the tones that represent the binary bit values aresomewhere between approximately 400 Hertz and approximately 1600 Hertz,and preferably between 500 and 900 Hertz. The IBS modulator 64 includesSine and Cosine tables that are used to generate the digital values thatrepresent the different amplitude and phase values for the f1 and f2frequencies.

[0064] In one embodiment of the invention, the digital data is output onthe radio channel 34 at a baud rate of 400 bits/second. This baud ratehas been found to be effective in preventing corruption of the digitalaudio data by a wide variety of different cellular telephone voicecoders. The sine waves for each f1 and f2 tone begin and end at a zeroamplitude point and continue for a duration of 10 milliseconds. Eightysamples are generated for each digital data tone.

[0065] Referring to FIG. 7, an Automatic Gain Controller (AGC) 80 is oneencoding function used in the cell phone 14. The AGC 80 may be softwarethat is located in the same DSP that implements the voice coder 18. TheAGC 80 scales instantaneous energy changes in voice signals. There aresituations when no voice signals have been fed into the AGC 80 for aperiod of time followed by a series of audio tones 82. that comprise thebeginning of a IBS packet 70. The AGC 80 scales the first group of tones82 at the beginning of the IBS packet 70. The AGC 80 also looks ahead atthe zero signal levels 84 after the end of the IBS packet 70, and willscale the tones 83 at the end of the IBS packet 70 as part of itsprediction scaling scheme. This scaling prevents the over amplificationof signal or noise when instantaneous energy changes occur in the voicechannel.

[0066] As previously shown in FIG. 6, the “1” and “0” bits of the IBSpacket 70 are represented by tones f1 and f2, respectively. If thesetones are scaled by the AGC 80, the digital bits represented by thosefrequencies might be dropped during encoding. For example, the vocoder18 may see the scaled tones as noise and filter them from the audiochannel. To prevent the unintentional filtering of tones that representdigital data, the IBS packet 70 in FIG. 5 includes preamble bits 72 andpostamble bits 79. The preamble bits 72 and postamble bits 79 do notcontain any of the digital data bits 29 from the data source but includea certain number of sacrificial bit(s) that are not needed for detectingor encoding the IBS packet 70. The tones that are generated for thesesacrificial bits in the preamble and postamble can be scaled or filteredby the AGC 80 without effecting any of the digital data contained in theIBS packet payload 76.

[0067] The bit pattern in the preamble 72 and sync pattern 74 arespecifically formatted to further prevent corruption of the packetpayload 76. A random sequence and/or an alternating “1”-“0” sequence ofbits is used in either the preamble 72 and/or sync pattern 74. Thesealternating or random bit patterns prevent adaptive filters in the cellphone vocoder 18 (FIG. 2) from filtering tones representing theremaining bits in the IBS packet 70.

[0068] Referring to FIG. 8, adaptive filters adapt around thefrequencies that are currently being transmitted over the wirelessnetwork. For example, if a long period of the same f1 tone is currentlybeing transmitted, an adaptive filter used in the cell phone may adaptaround that f1 frequency spectrum as shown by filter 86.

[0069] Another short tone at another frequency f2 may immediately followthe long period of f1 tones. If the filter 86 is too slow to adapt, thefirst few f2 tones may be filtered from the voice channel. If thefiltered f2 tone represent bits in the IBS bit stream, those bits arelost.

[0070] To prevent adaptive filters in the cell phone from dropping bits,some portion of the preamble 73 includes a random or alternating “1”-“0”bit pattern. This preconditions the adaptive filter as shown by filter88. The preamble 73 (FIG. 5) tries to include a portion of the same bitsequence that is likely or does occur in the packet payload 76. Forexample, the IBS encoder 52 can look ahead at the bit pattern in thepayload 76. The encoder 52 can then place a subset of bits in a portionof the preamble to represent the sequence of bits in the packet payload.

[0071] This preconditions the adaptive filter for the same f1 and f2frequencies, in the same duration and in a similar sequence that islikely to follow in the IBS packet payload 76. Thus, the adaptive filteris less likely to filter out the tones that actually represent thedigital data that is being transmitted.

[0072]FIG. 9 is a block diagram of receive circuitry 91 that receivesthe voice and data signals in the radio channel 34. The IBS modem 28also includes an IBS decoder 98 the detects and decodes the digital datatones transmitted in the radio channel 34. The receive circuitry 91 islocated at the CTSS 38 (FIG. 1) that receives wireless transmissionsfrom the cell sites 36 (FIG. 1). The same receive circuitry 91 is alsolocated in the cell phone 14.

[0073] As described above in FIGS. 2 and 3, the decoder part of the IBSmodem 28 can be external to the cell phone 14 or can be inside the cellphone 14. Dashed line 104 shows an IBS modem 28 external to a cell phoneand dashed line 106 shows an internal IBS modem 28 internal to a cellphone. IBS modems 14 can be located at any telephone location in thePSTN network 42 or IP network 46 (FIG. 1). The receiving circuitry 91may be different when the IBS modem 28 is coupled to a landline.However, the IBS modem 28 operates under the same principle bytransmitting and receiving synthesized tones over the voice channel ofthe phone line.

[0074] The signals in radio channel 34 are received by a transceiver 90.A vocoder 92 decodes the received signals. For example, the vocoder 92may decode signals transmitted in TDMA, CDMA, AMPS, etc. A D/A converter94 then converts the digital voice signals into analog signals. Theanalog voice signals are then output from an audio speaker 17.

[0075] If the IBS modem 28 is external to the receiving circuitry 91,then a A/D converter 96 converts the analog signals back into digitalsignals. The IBS decoder 98 demodulates any tones representing digitaldata back into a digital IBS packets. A packet disassembler 100disassembles the packet payload from the IBS packets 70 and stores thedecoded digital data in a data buffer 102.

[0076]FIG. 10 is a state diagram explaining how the IBS decoder 98 inFIG. 9 operates. The IBS decoder 98 repeatedly samples and decodes theaudio signals received from the radio channel 34. State 110 searches fortones in the audio signal that represent digital data. If the Signal toNoise Ratio (SNR), for tones within the frequency range of the digitaldata tones, are greater than a preselected value, the IBS decoder 98goes into an active state 112. The active state 112 collects tonesamples. If at any time during the active state 112 the SNR falls belowan active threshold value or a timeout is reached before enough tonesamples are collected, the IBS decoder 98 returns to the search state110 and begins again to search for digital data tones.

[0077] After a number of samples are collected, the IBS decoder 98 looksfor bits that identify the preamble 73 in the IBS packet 70 (FIG. 5). Ifthe preamble 73 is detected, the IBS decoder 98 moves to clock recoverystate 114. The clock recovery state 114 synchronizes with thesynchronization pattern 74 in the IBS packet 70 (FIG. 5). The IBSdecoder 98 then demodulates the packet payload 76 in state 116. If thepreamble 73 is not found, IBS decoder 98 goes back to the search state110 and starts searching again for the beginning of an IBS packet 70.

[0078] The IBS decoder 98 demodulates all of the packet payload 76 andthen performs a checksum 78 as a final verification that a valid IBSpacket 70 has been successfully demodulated. Control then returns backto the search state 110 and begins searching for the next IBS packet 70.

[0079]FIG. 11 is a detailed diagram for the search state 110 of the IBSdecoder 98. The search state 110 uses in band and out of band filtering.“In band” is used in the following discussion to refer to tones withinthe frequency range of the two tones that represent the digital databinary “1” value (500 Hz) and the digital data binary “0” value (600Hz).

[0080] A first band pass filter 118 (in band) measures energy forsignals in the audio channel within the frequency range of about 400 Hzto around 700 Hz. A second band pass filter 120 (out of band) measuresthe energy in the audio channel for signals outside of the 400 Hz-700 Hzrange. A Signal to Noise Ratio (SNR) is calculated in block 122 betweenthe in band energy and the out of band energy. If tones representing thedigital data exist in the audio channel, the energy measured by the inband filter 118 will be much greater then the energy measured by the outof band filter 120.

[0081] If the SNR is below a selected threshold in comparator box 124,signals in the audio channel are determined to be actual voice signalsor noise. If the SNR is above the threshold, the IBS decoder 98determines the tones represent in band digital data. When digital datais detected, the IBS decoder 98 moves into the active state 112 (FIG.10) to begin searching for the beginning of an IBS packet 70.

[0082]FIG. 12 shows the active state 112 for the IBS decoder 98. Block130 is notified by the search state 110 when an in band tone is detectedin the audio channel. Samples of the audio tones are windowed in block132 with a number of samples associated with a single binary bit. In oneembodiment, 80 samples of the digital data tone are taken, padded withzeros, and then correlated with Discrete Fourier Transforms (DFTs).

[0083] A first DFT has coefficients representing a 500 Hz tone and isapplied to the windowed data in block 134. The first DFT generates ahigh correlation value if the samples contain a 500 Hz tone (“0” binarybit value). A second DFT represents a 600 Hz tone and is applied to thewindowed samples in block 136. The second DFT generates a highcorrelation value if the windowed samples contain a 600 Hz tone (“1”binary bit value). Block 138 selects either a binary “0” or binary “1”bit value for the windowed data depending on which of the 500 Hz DFT or600 Hz DFT yields the largest correlation value.

[0084] The IBS decoder 98 in decision block 140 continues to demodulatethe tones until the preamble of the IBS packet 70 has been detected. TheIBS decoder 98 then moves to clock recovery state 114 (FIG. 13) tosynchronize with the sync pattern 74 in the IBS packet 70 (FIG. 5). Ifmore bits need to be demodulated before the preamble 73 can be verified,decision block 140 returns to block 132 and the next 80 samples of thedigital data tones are windowed and demodulated.

[0085]FIG. 13 describes the clock recovery state 114 for the IBS decoder98. After the preamble 73 in the IBS packet 70 is detected in the activestate 112, the clock recovery state 114 demodulates the next string ofbits associated with the sync pattern 74 (FIG. 5). The clock recoverystate 114 aligns the tone samples with the center of the correlationfilters described in the active state 112. This improves decoderaccuracy when demodulating the IBS packet payload 76.

[0086] Decision block 142 looks for the sync pattern 74 in the IBSpacket 70. If after demodulating the next tone, the sync pattern 74 isnot found, decision block 142 offsets the window used for sampling thesync pattern 74 by one sample in block 148. Decision block 150 thenrechecks for the sync pattern 74. If the sync pattern 74 is found,decision block 144 determines the power ratio for the detected syncpattern. This power ratio represents a confidence factor of how well thedemodulator is synchronized with the sync pattern. The power ratio iscompared with the power ratios derived for different window shiftedsampling positions. If the power ratio is greater then a previoussampling position, then that power ratio is saved as the new maximumpower ratio in block 146.

[0087] If the power ratio for the sync pattern 74 is less then thepreviously measured power ratio, the decoder in block 148 offsets thesampling window by one sample position. The power ratio is thendetermined for the shifted window and then compared to the currentmaximum power ratio in decision block 144. The window is shifted untilthe maximum power ratio is found for the sync pattern 74. The windowoffset value at the maximum power ratio is used to align the demodulatorcorrelation filters with the center sample of the first bit 77 (FIG. 5)in the IBS packet header 75.

[0088] The IBS decoder 89 then jumps to demodulate state 116 (FIG. 10)where the identified window offset is used to demodulate the remaining500 and 600 Hz tones that represent the packet payload bits 76 and checksum bits 78. The demodulation state 116 correlates the f1 and f2 toneswith DFTs in the same manner as in the active state (FIG. 12). The checksum bits 78 are then used as a final check to verify that a valid IBSpacket has been received and accurately decoded.

[0089]FIG. 14 is a diagram of the IBS modem 28 located in a battery packconnected to the cellular telephone 14. A hands free audio channel pin200 couples the IBS modem 28 to the voice channel 202 in the cell phone14. A switch 204 couples either voice signals from the microphone 17 ordigital data tones from the IBS modem 28 to the voice channel 202.

[0090] The switch 204 is controlled either through a menu on a screen(not shown) in the cell phone 14 or by a button 206 that extends out ofthe back end of the battery pack 208. The switch 204 can also becontrolled by one of the keys on the keyboard of the cell phone 14.

[0091] The button 206 can also be used to initiate other functionsprovided through the IBS modem 28. For example, a Global PositioningSystem (GPS) includes a GPS receiver 210 located in the battery pack208. The GPS receiver 210 receives GPS data from a GPS satellite 212. Acell phone operator simply pushes button 206 during an emergencysituation. Pressing the button 206 automatically enables the GPSreceiver 210 to collect GPS data from GPS satellite 212. At the sametime, the switch 204 connects IBS modem 28 on the voice channel 202 ofthe cell phone 14. The IBS modem 28 is then activated. As soon as theGPS data is collected in the IBS modem 28, the data is formatted,encoded and output by IBS modem 28 to the voice channel 202 of the cellphone 14.

[0092] The user 23 can push the button 206 anytime after manuallycalling up a phone number. After the audio channel is established withanother endpoint, the user 23 pushes button 206. Switch 204 is connectedto the IBS modem 28 and the IBS modem 28 is activated. The GPS data (orother digital source) is then sent as digital data tones through the IBSmodem 28 to an endpoint over the established audio channel. After thedata has been successfully transmitted, the user presses button 206again reconnecting switch 204 to the audio receiver 17.

[0093]FIG. 15 shows the different types of data sources that can beconnected to the IBS modem 28. Any one of a palm computer 212, GPSreceiver 214 or a computer 216, etc. can are coupled to the IBS modem28. The IBS modem 28 converts the bits output from the device intodigital data tones that are then output over the radio channel 34 in thewireless network. Because data can transmitted to another endpointthrough the cell phone 14, none of the devices 212, 214 or 216 need aseparate wireless modem.

[0094] Implementation of Inband Signaling Modem in a Sound Card

[0095] The IBS modem can be implemented in a standard computer soundcard. Referring to FIG. 16, a sound card 252, such as a Sound Blastercard manufactured by Creative Labs, Inc., 1523 Cimarron Plaza;Stillwater, Okla. 74075 is included in a computer 250. A speaker output253 of the sound card 252 outputs audio tones to a hands free port 257on a cell phone 258. A microphone input 259 on the sound card 252 isconnected to the speaker output of the cell phone 258.

[0096] The computer includes a processor 254 that converts digital datainto an audio format used by the sound card 252 to output synthesizedaudio tones. The cell phone 258 encodes and transmits those audio tonesover the voice channel of a wireless communications network. A cell site261 receives the transmitted audio tones and forwards the audio tonesover a PSTN network 263. A computer 262 is connected to a telephone line260 at the destination location of the phone call. Another sound card264 and a processor 266 in computer 262 demodulate the audio tones backinto digital data. The digital data represented by the audio tones aredisplayed on computer 262. The sound cards may be used for dataencoding, decoding or both. The sound cards may be used at computer 250,computer 262, or both.

[0097] Referring to FIGS. 16 and 17, data files, GPS data, data enteredby the keyboard by a user, or any other digital data is packetized andformatted by computer 250 into IBS packets in block 270. Packetizationand packet formatting is described in FIGS. 4 and 5. The binary bitvalues in the IBS packets are converted in block 272 into a digitalformat used by the sound card 252 (FIG. 16) for generating synthesizedaudio tones. For example, binary “1” bit values in the IBS packet areconverted into a digital format representing a first f1 frequency toneand binary “0” bit values are converted into a second f2 frequency tone.The f1 and f2 tones are generated similar to the manner described inFIG. 6.

[0098] The sound card in block 274 outputs analog tones representing thebinary bit values in a manner similar to the IBS encoder 52 and thedigital to analog converter 54 described in FIG. 3. The cell phone inblock 276 encodes the audio tones and transmits the encoded audio tonesover the voice channel in the wireless communications network in block278.

[0099] Referring to FIGS. 16 and 18, the cellular phone call isestablished with a destination phone number. In block 280, either a userpicks up the ringing phone line or the computer 262 (FIG. 16) at thedestination end of the cellular phone call is programmed to detect aringing signal from the telephone line 260. If a ring signal isdetected, either a user or the computer 262 in block 282 generates an“hook-off” signal on the telephone line 260. The sound card 264 in block284 acts like an analog to digital converter by converting the audiotones on the telephone line 260 into digital data. The sound card 264 inconjunction with the processor 266 (FIG. 16) decodes the IBS audio tonessimilar to the IBS decoder 98 described in FIGS. 9-13. The digitalrepresentations of detected IBS tones are then displayed on the screenof computer 262 in block 290.

[0100] In one example, a user wants to find the location for cell phone258. The user directs computer 262 (FIG. 16) to dial the phone numberfor cell phone 258. The computer 262 uses the sound card 264 to send IBStones that direct cell phone 258 to send back GPS location data. Thecomputer 250 may have a GPS receiver or the cell phone 258 may have astandalone GPS receiver. If the GPS receiver and the IBS modem areinternal to the cell phone 258 as shown in FIGS. 2-9, the computer 250does not need to be connected to the cell phone 258.

[0101] The GPS data is converted into IBS tones either by the sound card252 as described in FIG. 17 or through an internal IBS modem asdescribed in FIGS. 2-9. The IBS tones representing the GPS data aretransmitted back over the wireless telecommunications channel and thePSTN network 263 to the telephone line 260. The sound card 264 incomputer 262 monitors the phone line 260 for the IBS audio tones. Whendetected, the IBS tones are converted back into digital GPS data anddisplayed by processor 266 to the user on the screen of computer 262. Amapping process in the computer 262 may then convert the GPS longitudeand latitude values into a state, city and street address.

[0102] Synchronization

[0103]FIG. 19 shows an alternative technique for demodulating andsynchronizing the IBS modem in the IBS decoder 300. The IBS audio tonesare received over the voice channel of the wireless communicationsnetwork at interface 301. The received tones are converted from analogto digital form by A/D converter 302. The IBS signal detector 304detects the presence of the IBS audio tones in the same manner asdescribed in FIG. 11.

[0104] The alternative synchronization technique begins with the decoder300 tuning the IBS signals to complex basebands with multipliers 306 and308. Multiplier 306 effectively moves any IBS tones at the first andsecond IBS frequencies f1 and f2 to DC. This first baseband signal isreferred to as S_(A)′ and the second baseband signal is referred to asS_(B)′. A matched filter bank 310 applies matched filters to thebaseband signals having the expected pulse shapes for the two audiotones representing the binary “1” and binary “0” values. The S_(A)signal output from the matched filter bank 310 represents a binary 1value and the S_(B) signal represents a binary 0 value. The matchedfilter bank can also add filtering to account for known characteristicsof the wireless communications channel that may exist in the S_(A) orS_(B) signals.

[0105] The matched filter is selected to match the pulse shaping appliedto the modulator. The pulse shaping is selected for the best trade-offbetween signaling bandwidth, bit rate and inter symbol interference. Thepulse shaping filter is applied to the integrated phase of themodulator's numerical oscillator.

[0106] An IBS synchronizer 312 aligns the modulator with thesynchronization pattern attached to the front of the IBS packet.Segments 316 of samples from the S_(A) and S_(B) signals are input tosynchronization demodulator 314 along with a sample start time T_(B).The demodulator 314 outputs a power value 320 to the IBS synchronizer312 that indicates how closely the demodulator is synchronized with thebeginning bit in the synchronization pattern. The IBS synchronizer 312uses the power values 320 for each sample start time T_(B) to determinethe optimum synchronization start time (*T_(B)) for demodulating theremaining bits in the IBS packet. IBS packet modulator 322 then uses theoptimum start time *T_(B) to demodulate the binary bit values from theS_(A) and S_(B) signals.

[0107]FIG. 20 is a more detailed description of the sync demodulator 314and the IBS packet demodulator 322 in FIG. 19. A first integrator 324integrates the first segment of samples for the S_(A) signal. Theintegrator starts at sample start time T_(B) and integrates N number ofsamples representing the duration T of one IBS bit (Baud time). Arectifier 326 feeds the magnitude of the integration value into an adder332. In a similar matter, an integrator 328 integrates the segments ofsamples for signal S_(B) starting at sample start time T_(B). Arectifier 330 feeds the magnitude of the integrated segment of the S_(B)signal into adder 332. The output of adder 332 is a power signal 320that is fed back to the synchronizer 312. The IBS packet demodulator 322(FIG. 19) also includes a comparator 334 that generates either a binary1 value or a binary 0 value according to the magnitudes of the S_(A) andS_(B) signals.

[0108] To explain further, FIG. 21 shows a representation of the signalsS_(A) and S_(B) that are output from the matched filter bank 310. Anumber of samples 336 of the S_(A) or S_(B) signal represent the bitduration T of one IBS tone. In the example shown in FIG. 21, fivesamples are taken for each bit duration T. The sample start time T_(B)is shifted one sample for each integration. A starting sample for thefirst integration starts at sample start time T_(b1). As seen in FIG.21, the sample start time T_(b1) is not aligned with the S_(A) signalrepresenting a binary “1” value or the S_(B) signal representing abinary “0” value. The sync demodulator 314 in FIG. 20 generates a poweroutput value of 0.0 for T_(b1).

[0109] When sample start time T_(B2) is used, the demodulator 314generates an output value of −2.0. The sample start time T_(B3)represents the sample with the best synchronization with the beginningof the “0” tone in signal S_(B). At synchronization start time T_(B3)the output power is −3. As the sample start times T_(B4) and T_(B5) movefurther away from the best synchronization position, the magnitude ofthe output power decreases. FIG. 22 shows the magnitude of the powerdistribution for the different sample start times. The maximum powermagnitude is identified at sample start time T_(B3). Thus, the optimalsample start time T_(b3) is used by the IBS synchronizer 312 (FIG. 19).

[0110] Referring to FIGS. 20 and 21, a first sampling segment 338starting at sample time T_(b3) generates an output value from adder 332in FIG. 20 of −3. The comparator 334 in FIG. 20 generates a binary “0”value for any adder value less than zero. The output of adder 332 for asecond segment of sample values 340 generates an output value of +3.Because the output value for the second sample segment is greater than0, comparator 334 generates a binary “1” value. The IBS packetdemodulator 322 (FIG. 19) continues to decode the tones in the S_(A) andS_(B) signals for the remainder of the IBS bit stream.

[0111]FIG. 23 shows a variation of the synchronization scheme describedin FIGS. 19-22. The IBS tones are detected in block 341. The IBS tonesare shifted to baseband by the multipliers 342 for both the audio tonefrequency f_(A) representing a binary bit “1” value and for the audiotone f_(B) representing a binary bit “0”. The baseband shift is done foreach individual sample T(x) of the f_(A) and f_(B) signals

[0112] Instead of summing an entire baud of samples, a running sum ofthe latest baud value is taken using the new sample T(x) in block 344.For example, with a sample rate of 20 samples per bit, the 21^(st)sample T(N+1) is deleted from the running sum and the next sample T(x)is added to the running sum. The magnitude of the two running sums fortone A and tone B are each taken in blocks 345 and compared bycomparator 346. A binary “1” or binary “0” value is output fromcomparator 346 depending upon which of the A tone or B tone samples hasthe largest magnitude value. The binary bit values output fromcomparator 346 are correlated with the known sync pattern in thecorrelation block 347. The selected sample start time *T_(B) isidentified as the last sample that generates the largest correlationvalue with the synchronization pattern. The remaining bits in the IBSpacket are then demodulated according to the selected sample start time*T_(B).

[0113] Multichannel Inband Signaling Modem

[0114]FIG. 24 shows the encoder portion 350 of a Multichannel InbandSignaling (MIBS) modem. A data source 351 generates a binary bitstream.The MIBS encoder 350 generates multiple inband signaling channels withinthe same voice channel. A data buffer 352 stores the binary bit streamfrom the data source 351. A packet assembler 353 assembles the bits inbuffer 352 into a packet payload and adds a preamble and postamble tothe packet payload to form IBS packets as described above in FIG. 4.

[0115] The encoder 350 includes two modulators 356 and 362 that eachgenerate different audio tones that represent the bits in the IBSpackets. Modulator 356 modulates binary “1” values using an f1 frequency360 and modulates binary “0” values using an f2 frequency 358. Modulator362 modulates other bits in the IBS packets having binary “1” valuesusing an f3 frequency 364 and modulates binary “0” values using an f4frequency 366. The f1 and f2 tones output from modulator 356 arereferred to as a first Inband Signaling channel and the f3 and f4 tonesoutput from modulator 362 are referred to as a second IBS channel. Thetones output from the two modulators 356 and 362 are combined togetherby an adder 368 and then output to the D/A converter 370 and other cellphone circuitry 14 (FIG. 2). The cell phone circuitry 14 encodes andtransmits the tones in the two IBS channels over an audio channel of thecellular telephone network.

[0116] Each of the individual modulators 356 and 366 are similar inoperation to the IBS modulator 64 shown in FIG. 4. Any number of IBSchannels can be generated in the IBS modem 24. For example, a third IBSchannel could be provided by adding a third IBS modulator that modulatesbits for a third portion of the IBS packets into tones using frequenciesf5 and f6. The output of the third IBS modulator would be fed into theadder 368. However, for simplicity, only a two channel IBS modem withtwo corresponding IBS modulators 356 and 362 are shown in FIG. 24.

[0117] An IBS channel controller 354 controls how the multiple IBSchannels are utilized by the transmitting and receiving IBS modems. Forexample, a first 11S channel may only be used by a first IBS modem fortransmitting IBS packets and a second IBS channel may only be used bythat first IBS modem for receiving IBS packets. A second IBS modem onthe opposite end of the transmission then uses the second IBS channelfor transmission and uses the first IBS channel for reception. The IBSchannel controller 354 adds control bits into the IBS packets thatnegotiate use of the multiple IBS channels between the two communicatingIBS modems. The different configurations for the IBS modems aredescribed in further detail below in FIGS. 26 and 27. The controller 354also controls what portions of the IBS packets are modulated bymodulators 356 and 362. For example, the modulators may modulate everyother IBS packet or each modulator may modulate different portions ofthe same IBS packets.

[0118]FIG. 25 shows the decoder 375 of the MIBS modem. The audio tonesfrom the audio channel are decoded by receiving circuitry 372 and fedinto an A/D converter 374. A first filter 376 filters signals outside afrequency range of the two tones in the first IBS channel and a secondfilter 378 filters signals outside the frequency ranges of the two tonesin the second IBS channel. The frequency range of filter 376 is fromf1−Δf to f2+Δf and the frequency range of filter 378 is from f3−Δf tof4+Δf. The filters 376 and 378 are shown before the decoders 380 and382, respectfully. However, the filters 376 and 378 can be implementedin the same DSP anywhere in the decoding process.

[0119] A first IBS channel decoder 380 detects and demodulates the twotones in the first IBS channel into binary bit values and a second IBSchannel decoder 382 detects and demodulates the two tones in the secondIBS channel into binary bit values. The decoders 380 and 382 detect,synchronize, and demodulate the IBS tones in the same manner aspreviously described for decoder 98 in FIG. 9 or decoder 300 in FIG. 19.A packet assembler 386 assembles the bits output from the two decoders380 and 382 into IBS packets that are then output to a data buffer 388.

[0120] The IBS channel controller 384 in the receiving IBS modemsynchronizes the two decoders 380 and 382 and determines which decodersdemodulate what portions or which IBS packets. The controller 384 alsoconducts a communication protocol with the transmitting IBS modem thatnegotiates which IBS modem is transmitting and which IBS modem isreceiving IBS packets over which IBS channels.

[0121] The filter 376 and decoder 380 for the first IBS channel and thefilter 378 and decoder 382 for the second IBS channel can be implementedin software in the same DSP. Alternatively, one DSP can be used for eachindividual channel encoder and decoder in each MIBS modem.

[0122] It is preferred in the “MIBS” modem for frequencies f1 & f2 to befar apart from frequencies f2 and f3. One advantage of MIBS isinterference mitigation and the ability to adapt to variations in cellphone performance across manufacturers by dynamically changingfrequencies when performance is bad. A robust low baud rate controlsignal can be sent to choose a new frequency when one modem is detectingerrors.

[0123]FIG. 26 shows one possible configuration for two MultichannelInband Signaling (MIBS) modems 390 and 396. The two IBS channels 398 and400 are transmitted from MIBS modem 390 over the voice channel of awireless communications network and then possibly through a landlinetelephone network to the MIBS modem 396. The two MIBS modems shown inFIG. 26 operate in a half duplex mode where one of the IBS modemstransmits IBS packets over both the first IBS channel 398 and the secondIBS channel 400 at the same time.

[0124] After the first IBS modem 390 has completed a transmission 392 ofIBS packets over the two IBS channels, the second IBS modem 396 isallowed to begin a transmission 394 back to modem 390 over the two IBSchannels 398 and 400. The MIBS modem 390 sends information in one of theIBS packets indicating to the MIBS modem 396 that the transmission 392is completed.

[0125]FIG. 27 shows an alternative configuration where the first IBSchannel 398 is dedicated to transmitting IBS packets from MIBS modem 390and the second IBS channel 400 is dedicated to transmitting packets fromMIBS modem 396. Thus, both MIBS modem 390 and 396 can transmit andreceive packets at the same time. This full duplex configuration canprovide faster communications for certain types of IBS transmissions.

[0126] The MIBS modem 390 may transmit different potions of the same IBSpackets over the two IBS channels 398 and 400 or may alternatetransmission of different IBS packets over the two IBS channels. Inother configurations, one IBS channel may be used for transmitting IBSpackets and the second IBS channel may be used exclusively for signalingand protocol communications between the two MIBS modems. In otheralternative configurations, portions of bits from the same IBS packetsare interleaved in the two IBS channels or the same IBS packets aretransmitted over both IBS channels for redundancy. The information inthe two IBS channels can be reconfigured according to the applicationassociated with IBS packet data.

[0127] A request to reconfigure the IBS channels can be encoded into theIBS packet header. For example, the IBS channel controller 354 (FIG. 24)in MIBS modem 390 may send an IBS packet to MIBS modem 396 that containsa reconfiguration request in the IBS packet preamble 73 (FIG. 5). Thereconfiguration request from modem 390 may request both the first IBSchannel 398 and the second IBS channel 400 and then request allocationof a third IBS channel 401, with a slower baud rate, to MIBS modem 396for transmitting acknowledge messages back to modem 390. MIBS modem 390then waits for an acknowledge of the configuration request from modem396.

[0128] The IBS channel controller 384 (FIG. 25) in MIBS modem 396 readsthe reconfiguration request in the IBS packet preamble. The controller384 then outputs an acknowledge back through the encoder of MIBS modem396. The encoder formats the acknowledge into the preamble of a replyIBS packet that is then modulated and transmitted back to MIBS modem 390over one or more of the currently allocated IBS channels. The controllerin the modem 396 then reconfigures the encoder to receive IBS packetsover the first and second IBS channels 398 and 400 and transmit packetsover the third low baud rate channel 401.

[0129] When the acknowledge from modem 396 is received at modem 390, thecontroller directs the encoder and the decoder in the modem 390 totransmit over the first and second IBS channels and receive from the lowbaud rate third channel. The two modems 390 and 396 then transmits andreceive IBS packets according to the new channel configuration.

[0130] Multicarrier Inband Signaling Modem

[0131]FIG. 28 shows a Multicarrier Inband Signaling modem according toanother aspect of the invention. The multichannel IBS modem described inFIGS. 24-27 generates two different audio tones, one tone representing abinary “1” value and a second tone representing a binary “0” value. Thetwo tones are generated in a sequential tone stream over time torepresent a binary bit stream.

[0132] The multicarrier IBS modem in FIG. 28 generates multiple audiotones at the same time, where each tone represents a different bitlocation in a four bit portion of the IBS packet. The particular audiotone associated with one of the four bit locations represents a binary“1” value (or alternatively a binary “0” value). If the audio tone isnot generated for a particular bit time (baud), the IBS decoder assumesthe binary bit value associated with that bit location is “0”.

[0133] Referring to FIG. 28, a stream of bits are input to data buffer402 for transmission over the audio channel of a wireless communicationsnetwork. A packet formatter 404 formats those bits into an IBS packet. Afirst portion of one of the IBS packets contains the bits “1010”. Thepacket formatter 404 outputs each one of the four bits into a differentone of the four modulators 406, 408, 410 and 412. The first bit “1” ofthe four bit sequence is referred to as bit B1, the second bit “0” isreferred to as bit B2, the third bit “1” of the four bit sequence isreferred to as bit B3, and the fourth bit “0” is referred to as bit B4.

[0134] Modulator 406 receives bit B1, modulator 408 receives bit B2,modulator 410 receives bit B3, and modulator 412 receives bit B4.Because bit B1 is a binary “1” value, modulator 406 generates a tone atfrequency f1 during the first baud period. The modulator 408 does notgenerate an f2 tone for the first baud period because the B2 bit is abinary “0” value. Accordingly, modulator 410 generates a f3 tone duringthe first baud period and modulator 412 does not generate a f4 toneduring the first baud period. The modulators work in essentially thesame manner as the IBS modulator 64 in FIG. 4 except that a frequencytone is generated for the binary “1” values and no tone is generated forthe binary “0” value.

[0135] The f1 and f3 tones are combined together by summer 414. Adigital to analog converter 416 converts the digital signal into ananalog signal that is fed into cell phone transmit circuitry 418. Thetransmit circuitry 418 transmits the audio tones over the voice channelof the cellular telephone network.

[0136]FIG. 29 shows the decoder for the multicarrier IBS modem. Receivecircuitry 420 receives the IBS tones from the voice channel of thecellular communications network. An A/D converter 422 converts the audiotones into a digital signal. Four bandpass filters 424, 426, 428 and 430each are centered about the frequency for the tones f1, f2, f3, and f3,respectively. The tone representing the binary bit B1 passes throughbandpass filter 424 while other tones, such as tone f3, are filtered bythe bandpass filter f1. Decoder 432 identifies the tone f1 in a mannersimilar to the IBS decoder described in FIGS. 11-13, only for a singletone. Because the f1 tone was detected by decoder 432, a binary “1”value is generated representing bit B1 in the four bit sequence.

[0137] Because no f2 tone will be detected by decoder 434, a binary “0”value is generated for bit B2 in the four bit sequence. Decoder 436detects an f3 tone and accordingly generates a binary “1” value for bitB3. Decoder 438 generates a binary “0” value for bit B4 because no f4tone was generated by the multicarrier encoder. A packet assembler 440receives the four bits B1-B4 and places them into the appropriate IBSpacket location in the data buffer 442.

[0138] Clock Synchronization Using IBS Demodulator

[0139] Referring to FIG. 30, a communications network 99 includes amobile station 100 that includes a mobile clock 101. The mobile station100 in one embodiment is a cellular telephone, or other type of mobiledevice, that includes a Global Positioning System (GPS). The mobilestation 100 receives global positioning data from satellite 102. Themobile station 100 receives position data from the satellite 102 andtransmits the position data to a reference station 108. The referencestation 108 may then forward the location data to another destination ina communications network 112. The communications network 112 can includea Public Services Telephone Network (PSTN) and an Internet Protocol (IP)network.

[0140] The reference station 108, or some other location in thecommunications network 112, includes a reference clock 110. Wirelessradio communications between the mobile station 100 and the referencestation 108 may use either analog or digital signaling. The referencestation 108 and the mobile station 100 may transmit data or voicesignals.

[0141] A synchronization flag (sync flag) 104 is used to synchronize themobile clock 101 in the mobile station 100 with the reference clock 110at reference station 110. The sync flag 104 contains a sequence of bitsthat are identified by the mobile station 100. The sync flag 104 isreceived by the mobile station 100 then sent back to the referencestation 108.

[0142] The sync flag 104 includes a reference bit 105 associated with aspecific time when the sync flag was transmitted. After thesynchronization flag 104 is returned by the mobile station 100 back tothe reference station 108, the IBS modem 28 in reference station 108synchronizes with the returned sync flag 104 in one of the waysdescribed in FIG. 13 or FIGS. 19-23. The reference station 108 thenidentifies a final time associated with the returned reference bit 105.

[0143] By first synchronizing the IBS modem 28 with the sync flag 104,the reference station 108 can more accurately determine the networkdelay in communications system 99. The round trip delay is defined as anamount of time required for the sync flag 104 to go from referencestation 108, to mobile station 100 and back to reference station 108.The round trip delay is divided by two to derive the one-way networkdelay time. The network delay time can then be subtracted from areference time 107 by either the mobile station 100 or the referencestation 108 to compensate for network delay. The mobile station 100 usesthe adjusted reference time to update the mobile clock 101. Depending onthe application, the IBS modem 28 in the mobile station 100, or the IBSmodem 28 in reference station 108, or both, may synchronize with thesync flag 104 and determine a reference time according to the referencebit.

[0144]FIG. 31 shows a single synchronization pulse 120 used forcalculating network delay. Network delay is determined by calculatingthe amount of time required for the pulse 120 to go from the referencestation, to the mobile station and then back at the reference station.

[0145] The problem with using a single reference pulse 120 is that thepulse may be distorted by encoding and decoding circuitry in thecommunications network. In one particular application, the referencepulse 120 is transmitted over the same channel in the communicationsnetwork used for transmitting and receiving voice signals. In thisapplication, a voice codec may filter out or further corrupt thereference pulse 120.

[0146] A reference pulse 122 shows one example of distortion. Becausepulse 122 is distorted, the mobile station and/or the reference stationcannot accurately determine when the pulse 122 is received. Thisprevents an accurate determination of the network delay and preventaccurate synchronization of the mobile clock with the reference clock.

[0147] To explain further, a number of samples 123 are taken by thereference station of the reference pulse 120. The sample rate may besome value such as 125 microseconds. A detected transition in the pulse120 from a zero value to some threshold value, is identified asreference point 124. The reference point 124 is used by the referencestation to calculate the network delay time.

[0148] If the pulse 120 is corrupted, as shown by pulse 122, then thedetected reference point 126 may shift in time from the originallytransmitted reference point 124. Because the initial portion of thepulse 122 is corrupted., the detected reference point 126 is offset 3sample locations from the original reference point 124. This results ina 125 microsecond/sample×3 samples=375 microsecond timing offset. Thistiming offset reduces the precision of the calculated round trip delaytime.

[0149]FIG. 32 shows how the sync flag 104 prevents signal corruptionfrom effecting the detected reference point and, in turn, effecting thecalculated network delay time. Instead of transmitting a singlereference pulse 120 (FIG. 31), a sequence of sync bits are transmittedin the sync flag 104. One of the bits in the sync flag 104 is used asthe reference bit 105. An initial start time T₀ for the reference bit105 is tracked by the reference station. In the example shown in FIG.32, the transition from a “0” bit to the first “1” bit in the sync flag104 is selected as the T₀ reference point 132.

[0150] Instead of simply looking for a single signal threshold change ina single reference pulse as shown in FIG. 31, the IBS modem firstsynchronizes with the entire sync flag 104 as shown by block 130.Several implementations of this modem synchronization are described inFIGS. 10, 13 and 19-23. The modem synchronization shifts a sample starttime 136 for the entire sync flag 104 until an optimum power ratio isgenerated. By identifying an optimum power ratio for the entire syncflag 104, a few corrupted samples in the sync flag 104 are less likelyto adversely effect the detected location of reference point 132.

[0151] Dashed line 134 represents signal corruption in the sync flag104. Because the IBS modem in block 130 synchronizes with multiple bitsof the synchronization flag 104, the corrupted samples 134 in thesynchronization flag 104 do not offset where the reference point 132 isdetected. In a general case, the reference point 132 will be no morethen +/−one half of the sample rate. In the example shown here, thereference point 129 will be off no more then +/−62.5 microseconds.

[0152] In one variation, multiple sync flags 104 are sent. This ensuresthat the mobile station will detect at least one of the sync flags 104.The reference time is adjusted according to which sync flag is used. Inanother aspect of the invention, different tone frequencies aregenerated by the IBS modem 28 to represent the binary “1” values and thebinary “0” values in the sync flag 104. These tones are transmitted overthe audio channel of the communications network. This is described abovein FIGS. 1-29. By generating tones having frequencies within a humanvoice range, voice codecs in the communications network are less likelyto corrupt or filter the binary bits represented by the generated tones.

[0153] The individual bits in each sync flag 104 have a sorter durationthan the single synchronization pulse 120 shown in FIG. 31. Thisprovides a better signaling environment for properly passing the bitsthrough digital voice coders in wireless networks. Thus, using the syncflag 104 instead of using a single sync pulse further reduces signalcorruption.

[0154] Referring to FIG. 33, the sync flag 104 is initially sent out bythe reference station in step 140. The mobile station receives the syncflag and then returns the sync flag back to the reference station instep 142. The IBS modem in the reference station synchronizes with thereturned sync flag in step 144. As described in FIG. 13 and FIGS. 19-23,the IBS modem synchronizes with the sync flag by varying the samplestart time (T_(B)) until the optimum power ratio for the synchronizationflag is determined.

[0155] After the IBS modem is synchronized, block 146 uses the optimumsample start time (*T_(B)) to determine the time associated withreference point 132 (FIG. 32) in the sync flag. The time associated withthe reference point 132 is determined by multiplying the number ofsamples between the optimum sample start time (*T_(B)) and the referencepoint 132 by the sample rate. The time from when the reference bit wasoriginally output by the reference station (T₀) to when the referencebit is received back by the reference station (T_(final)) is the roundtrip delay time. Block 148 determines the round trip delay and dividesthe round trip delay in half to determine the one way time delay in thenetwork (network delay). The derived network delay is used in block 150to synchronize the mobile clock 101 (FIG. 30) or adjust any timedependant data in the network.

[0156] An alternative implementation of the sync flag is shown in FIG.34. A data packet 152 is used for identifying network delay. A preamble154 identifies the packet 152 as a clock recovery packet. Payload data156 in the clock recovery packet 152 uses a preselected format andsequence of bits for modem synchronization. For example, the second bit158 in the payload 156 is used as the reference bit. The IBS modem firstsynchronizes with the group of synchronization bits 160 in the payload156. An optimum sample start time is identified for the sync bits 160.Then a reference time is determined according to the reference bit 158in the synchronization bits 160.

[0157] Additional information can be incorporated into the packet 152for more accurately identifying network delay. For example, a second setof bits 162 in the payload 156 identify the turn around time required bythe mobile station 100 (FIG. 30) to process the synchronization packet152. The mobile station 100 determines when the packet 152 is firstreceived. The mobile station 100 then calculates when the packet 152 istransmitted back to the reference station 108. The time from when thepacket 152 is first received by the mobile station to when the replypacket is transmitted back to the reference station is referred to asthe turn around time. The turn around time 162 can be subtracted fromthe round trip time to more accurately determine how long it takes apacket to travel between the reference station and the remote station.

[0158] Bits can be padded in payload 156 to ensure approximately thesame turn around time for each synchronization packet 152. By using thepacket 152, other payload data 164 can also be carried along with thesync bits 160 and 162. The synchronization packet 152 can beincorporated into a wider variety of network protocols since the timingcommunications are performed at one level higher than thesynchronization flag scheme described in FIG. 30.

[0159] Having described and illustrated the principles of the inventionin a preferred embodiment thereof, it should be apparent that theinvention can be modified in arrangement and detail without departingfrom such principles. I claim all modifications and variation comingwithin the spirit and scope of the following claims.

1. A system for identifying transmission delay in a communicationnetwork comprising: a data source adapted to generate a sequence ofbits; a demodulator adapted to synchronize with the sequence of bits byshifting samples of the sequence of bits until an optimumsynchronization value is generated; and a synchronizer adapted toidentify a network delay value according to the synchronized sequence ofbits.
 2. A system according to claim 1 including an encoder adapted toconvert the sequence of bits into tones.
 3. A system according to claim2 wherein the demodulator synchronizes with the sequence of bits byshifting samples of the tones until a maximum power ratio is detectedfor a bit pattern associated with the sequence of bits.
 4. A systemaccording to claim 1 including a packet formatter adapted to format thesequence of bits into a packet having a preamble that identifies thepacket as a synchronization packet.
 5. A system according to claim 4wherein the packet formatter inserts bits in the packet that identify aturn around time representing an amount of time required to receive thesynchronization packet and then output a reply synchronization packet.6. A system according to claim 2 wherein the encoder converts binary “1”bits in the sequence of bits to a first tone having a first audiofrequency and converts binary “0” bits in the sequence of bits to asecond tone having a second audio frequency.
 7. A system according toclaim 6 wherein the demodulator compares samples of the first audio tonewith samples of the second audio tone and synchronizes with the sequenceof bits by shifting a sample start time for the samples of the first andsecond audio tones until the optimum synchronization value is detected.8. A system according to claim 7 wherein the network delay value isidentified according to a reference bit in the sequence of bits locatedaccording to the sample start time at the optimum synchronization value.9. A system according to claim 2 including: a digital to analogconverter that converts the tones for the sequence of bits into analogsignals; and a voice codec that encodes the analog signals fortransmission over a voice channel in the communications network.
 10. Asystem according to claim 9 wherein at least a portion of thecommunications network is a wireless cellular network and the voicecodec encodes both voice signals and the analog signals for the sequenceof bits for transmission over the voice channel in the wireless cellularnetwork.
 11. A method for identifying network delay, comprising:receiving tones that represent a sequence of bits, one of the bitsidentified as a reference bit; sampling the tones beginning at aselected sample start time; demodulating the tone samples to identifythe bit values in the synchronization flag; synchronizing with the tonesamples by shifting the sample start time until the tone samplesgenerate an optimum synchronization value; and deriving a reference timeaccording to the reference bit at the optimum synchronization value. 12.A method according to claim 11 including: identifying an initial timewhen the reference bit is first transmitted to a mobile station;receiving the sequence of bits back from the mobile station;sychronizing with the sequence of bits; identifying a final time whenthe reference bit is received in the synchronized sequence of bits; andderiving a delay time by comparing the initial time with the final time.13. A method according to claim 11 wherein the tones are received over avoice channel in a wireless cellular network.
 14. A method according toclaim 11 including: sampling the tones for a first frequencyrepresenting a binary 1 value; sampling the tones for a second frequencyrepresenting a binary 0 value; generating synchronization values bycomparing the tone samples for the frequency with the tone samples forthe second frequency; and shifting the sample start time for the samplesof the first tone and second tone until the optimum synchronizationvalue is derived.
 15. A method according to claim 14 including derivingthe reference time by identifying one of the tone samples at the optimumsynchronization value associated with the reference bit.
 16. A methodaccording to claim 11 including formatting the sequence of bits into apacket and synchronizing with the sequence of bits in the packet.
 17. Amethod system according to claim 16 including inserting bits in thepacket that identify a turn around time representing an amount of timerequired to process the packet containing the sequence of bits.
 18. Acomputer readable medium containing code for identifying network delay,comprising: code adapted to receive tones that represent a sequence ofbits; code adapted to sample the tones beginning at a selected samplestart time; code adapted to demodulate the sampled tones back into bitvalues representing the sequence of bits; code adapted to synchronizewith the sequence of bits by shifting the sample start time until thesampled tones generate an optimum synchronization value; and codeadapted to derive a reference time according to the sample start time atthe optimum synchronization value.
 19. Code according to claim 18including: code adapted to identify an initial time when the sequence ofbits are first transmitted; code adapted to receive the sequence of bitsback from a remote station; code adapted to sychronize with the returnedsequence of bits; code adapted to identify a final time according to thesynchronized returned sequence of bits; and deriving a network delaytime by comparing the initial time with the final time.
 20. Codeaccording to claim 18 wherein the tones are received over a voicechannel in a network.
 21. Code according to claim 18 including: codeadapted to sample the tones for a first frequency representing a binary1 value; code adapted to sample the tones for a second frequencyrepresenting a binary 0 value; code adapted to generate synchronizationvalues by comparing the samples for the first tone frequency with thesamples for the second tone frequency; and code adapted to shift thesample start time for the samples of the first tone frequency and secondtone frequency until the optimum synchronization value is derived. 22.Code according to claim 18 including: code adapted to receive a packethaving a preamble that identifies the sequence of bits; and code adaptedto synchronize with the sequence of bits in the packet.
 23. Codeaccording to claim 22 including code that inserts bits in the packetthat identify a turn around time representing an amount of time requiredto process the packet containing the sequence of bits.